Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains.
1. Field of the Invention
The present invention relates generally to optical traps. In particular, the invention relates to an apparatus, system and method for applying optical gradient forces to form a plurality of optical traps to manipulate small particles.
2. Discussion of the Related Arts
An optical tweezer is an optical tool which utilizes the gradient forces of a focused beam of light to manipulate particles with dielectric constants higher than the surrounding media. To minimize its energy such particles will move to the area where the electric field is the highest. Stated in terms of momentum, the focused beam of light produces radiation pressure, creating small forces by absorption, reflection, diffraction or refraction of the light by a particle. The forces generated by radiation pressure are almost negligible—a light source, such as a diode-pumped Nd:YAG laser operating at 10 mW, will only produce a few picoNewtons. However, a few picoNewtons of force is sufficient to manipulate small particles.
Other optical tools which can be used to manipulate small particles include, but are not limited to, optical vortices, optical bottles, optical rotators and light cages. An optical vortex, although similar in use to an optical tweezer, operates on a different principle.
An optical vortex produces a gradient surrounding an area of zero electric field which is useful to manipulate particles with dielectric constants lower than the surrounding media or which are reflective, or other types of particles which are repelled by an optical tweezer. To minimize its energy such a particle will move to the area where the electric field is the lowest, namely the zero electric field area at the focal point of an appropriately shaped laser beam.
The optical vortex provides an area of zero electric field much like the hole in a doughnut (toroid). The optical gradient is radial with the highest electric field at the circumference of the doughnut. The optical vortex detains a small particle within the hole of the doughnut. The detention is accomplished by slipping the vortex over the small particle along the line of zero electric field.
The optical bottle differs from an optical vortex in that it has a zero electric field only at the focus and a non-zero electric field at an end of the vortex. An optical bottle may be useful in trapping atoms and nanoclusters which may be too small or too absorptive to trap with an optical vortex or optical tweezers. J. Arlt and M. J. Padgett. “Generation of a beam with a dark focus surrounded by areas of higher intensity: The optical bottle beam,” Opt. Lett. 25, 191–193, 2000.
The optical rotator provides a pattern of spiral arms which trap objects. Changing the pattern causes the trapped objects to rotate. L. Paterson, M. P. MacDonald, J. Arlt, W. Sibbett, P. E. Bryant, and K. Dholakia, “Controlled rotation of optically trapped microscopic particles,” Science 292, 912–914, 2001. This class of tool may be useful for manipulating non-spherical particles and driving MEMs devices or nano-machinery.
The light cage, (Neal in U.S. Pat. No. 5,939,716) is loosely, a macroscopic cousin of the optical vortex. A light cage forms a time-averaged ring of optical tweezers to surround a particle too large or reflective to be trapped with dielectric constants lower than the surrounding medium. If the optical vortex is like a doughnut, the light cage is like a jelly-filled doughnut. While the doughnut hole (for the vortex) is an area of zero electric field, the jelly-fill is an area of lowered electric field. In a gross sense, the gradient forces of the plurality of optical tweezers forming the doughnut “push” a particle, with a dielectric constant lower than the surrounding medium, towards the jelly-fill which may also be thought of as the less bright area which lies between the plurality of optical tweezers. However, unlike a vortex, no-zero electric field area is created. An optical vortex, although similar in use to an optical tweezer, operates on an opposite principle.
Using a single beam of laser light with a diffractive optical element to form a plurality of diffracted laser beams focused to form an array of optical traps is known in the art. U.S. Pat. No. 6,055,106 issued to Grier and Dufresne describes arrays of optical traps. The Grier and Dufresne patent teaches the use of a dynamic optical element and a focusing lens to diffract the input light beam and generate an array of movable optical traps. The array of optical traps is formed from a single input beam by having an appropriate shape at the back aperture beam diameter. Specifically, a gaussian TEM00 input laser beam should have a beam diameter which substantially coincides with the diameter of the back aperture.
One limitation of having the beam diameter of a gaussian TEM00 input laser beam substantially coincides with the diameter of the back aperture is that as shown from a cross sectional view (FIG. 1) a gaussian TEM00 beam has much less intensity at its periphery The resulting optical traps will have a similar cross section of intensity.
Accordingly, there has existed a need to have an input beam fill the back aperture and produce optical traps with greater intensity at the periphery. The present invention satisfies these and other needs, and provides further related advantages.